Prior Art
The Following is a tabulation of prior art that appear to be relevant:
U.S. PatentsPat. No.Kind CodeIssue DatePatentee9,206,710A2015 Dec. 8Gurin9,145,795A2015 Sep. 29Lehar9,115,603A2015 Aug. 25Leibowitz9,051,852A2015 Jun. 9Geskes9,021,808A2015 May 5Nelson9,003,798A2015 Apr. 14Yanagi8,674,525A2015 Mar. 18Van Den Bossche8,387,386A2015 Mar. 18Schmeltz8,225,609A2015 Jul. 24Hinderling
Foreign Patent DocumentsPublication Doc. Nr.Cntry CodeBubl. DateApplicant176812WONovember 2015KOLLMEIER
Non Patent Literature Documents
Thermodynamic power cycles are used to convert thermal energy into mechanical energy. In a power cycle, a working fluid flows through a series of thermodynamic processes to achieve this energy conversion. An example of this type of cycle is a prior art Rankine cycle. In a prior art Rankine cycle, a typical working fluid is water.
The thermodynamic state of a working fluid can be described by specifying a set of thermodynamic properties. Two properties are needed to defined the state of a working fluid. If two properties are known, the state of the working fluid is defined and as a result, all of the remaining properties are known. Examples of thermodynamic properties include: temperature, pressure, mass density, internal energy, enthalpy, and entropy.
A working fluid can exist in one of the following phases: solid, liquid or vapor. In the liquid phase, a working fluid can exist as a saturated liquid or compressed liquid. In the vapor phase, the working fluid can exist as a saturated vapor or superheated vapor. If a working fluid is a saturated fluid, the fluid can exist as a solid, liquid or vapor. In addition, the working fluid can simultaneously exist as a saturated solid and liquid, a saturated liquid and vapor, and a saturated solid, liquid and vapor. If a saturated fluid is at a given temperature, the saturation pressure is known and as a result, the remaining thermodynamic properties are known.
Prior Art Rankine Cycle FIG. 1A and FIG. 1B
FIG. 1A shows an example of a prior art Rankine cycle 500, wherein a representative thermodynamic cycle is described. At the beginning of the cycle, the working fluid starts as a saturated liquid 10. The working fluid is then pressurized to high pressure compressed liquid 12 by pump 30. Input work 120 is the work required from pump 30 to increase the pressure from saturated liquid 10 pressure to compressed liquid 12 pressure. After leaving pump 30, compressed liquid 12 enters evaporator 40. High temperature input thermal energy 100 enters evaporator 40 and is transferred to compressed liquid 12 to produce high pressure and temperature vapor 14. The vaporization process from compressed liquid 12 to high temperature and pressure vapor 14 is a constant pressure process. In other words, compressed liquid 12 and superheated vapor 14 exist at the same pressure. High pressure and temperature superheated vapor 14 exits evaporator 40 and flows into expander 50. Expander 50 may be a positive displacement machine of various configurations, including but not limited to a screw expander or a turbine. In expander 50, high pressure and temperature vapor 14 expands to low pressure and temperature saturated vapor 16 at the exit of expander 50. This expansion produces output work 200 in the form of rotational kinetic energy that is operatively coupled to drive an electrical generator to produce electrical power. The electrical power may be delivered to a local isolated power grid or a commercial power grid. Energy that is transferred from saturated vapor 16 and saturated liquid 10, while transiting condenser 60, is waste thermal energy 150 and is expelled to the environment by condenser 60. The transfer of waste thermal energy 150 from the working fluid results in condensation of saturated vapor 16 back to saturated liquid 10. Waste thermal energy 150 can also be described as the working fluid's latent heat of condensation. Saturated liquid 10 then flows from condenser 60 and reenters pump 30 to repeat the cycle.
A sufficient temperature difference between the working fluid and environment is required for waste thermal energy 150 to be expelled to the environment. This means that the working fluid saturation temperature is substantially determined by the environmental temperature. For a saturated vapor, definition of the saturation temperature is sufficient to determine the remaining thermodynamic properties including pressure, mass density, enthalpy, internal energy and entropy.
The expansion of high temperature and pressure vapor 14 to saturated vapor 16 in expander 50 is substantially a constant entropy adiabatic expansion process. As a result, the entropy of saturated vapor 16 is substantially the same as the entropy of high temperature and pressure vapor 14. In addition, the temperature of high temperature and pressure vapor 14 is determined by the temperature of input thermal energy 100. This means the pressure of high temperature and pressure vapor 14 is determined by the entropy of saturated vapor 16 and the temperature of input thermal energy 100.
FIG. 1B shows a temperature vs. entropy diagram for prior art Rankine cycle 500. Saturation dome 20 is comprised of critical point 21, saturated liquid transition 22, and saturated vapor transition 25. Saturated liquid transition 22 represents the transition from compressed liquid region 24 to saturated fluid region 25. Any temperature and entropy coordinate that falls on saturated liquid transition 22 represents a working fluid that is in a saturated liquid state. Saturated fluid region 25 represents temperature and entropy values where saturated liquid and saturated vapor can coexist. Saturated vapor transition 26 represents the transition from saturated fluid region 25 to superheated vapor region 28. Any temperature and entropy coordinate that falls on saturated vapor transition 26 represents a working fluid that is in a saturated vapor state. Critical point 21 represents where saturated liquid transition 22 and saturated vapor transition 26 meet. The working fluid at any temperature above critical point 21 is a superheated fluid. As FIG. 1B shows, saturated liquid 10 falls on saturated liquid transition 22, compressed liquid 12 is positioned within compressed liquid region 24, high temperature and pressure vapor 14 is positioned within superheated vapor region 28, and saturated vapor 16 falls on saturated vapor transition 26.
Constant entropy process 55 represents the adiabatic process of expanding high temperature and pressure vapor 14 to saturated vapor 16. Because of constant entropy process 55 the minimum and maximum temperature limits for prior art Rankine cycle 500 are control by the saturation conditions of saturated vapor 16. The minimum temperature limit and the entropy are determined by the temperature of saturated vapor 16. The saturation temperature is determined by the temperature of the environment to which waste thermal energy 150 can be expelled. The entropy of constant entropy process 55 is the entropy of saturated vapor 16 at the saturation temperature. The maximum temperature limit of high temperature and pressure vapor 14 is determined by the temperature limit of input thermal energy 100. With the temperature of input thermal energy 100 and saturated vapor 16 entropy, the remaining thermodynamic properties of high temperature and pressure vapor 14 can be determined including pressure and enthalpy.
The vaporization of compressed liquid 12 to high pressure and temperature vapor 14 is represented by constant pressure process 45. As the temperature of compressed liquid 12 increases, the pressure remains constant. Vaporization of the working fluid between saturation liquid transition 22 and saturated vapor transition 26 is a constant temperature and pressure process. As the temperature of the working fluid increases to high temperature and pressure vapor 14, the pressure also remains constant. As a result, constant pressure process 45 extends from compressed liquid 12 to high temperature and pressure vapor 14. This means that entropy of constant entropy process 55 and the temperature of high temperature and pressure vapor 14 determine the pressure of constant pressure process 45.
The condensation of saturated vapor 16 to saturated liquid 10 is a constant pressure and temperature process that is represented by condensation process 65. Condensation process 65 extents from saturated vapor 16 to saturated liquid 10. The temperature and pressure of condensation process 65 are the saturation pressure and temperature of the working fluid.
In an alternate embodiment of strain augmented power cycle 600, the constant entropy process could intersect the condensation process inside the saturated fluid region. An example of this alternate process is represented by alternate constant entropy process 56. In this process, alternate constant entropy process 56 crosses saturated vapor transition 26 and intersects condensation process 65 inside saturated fluid region 25.
Output work 200 can be calculated from the difference between the enthalpy of superheated vapor 14 and the enthalpy of saturated vapor 16. The input thermal energy 100 can be calculated by the difference between the enthalpy of superheated vapor 14 and the enthalpy of compressed liquid 12. The enthalpy of compressed liquid can be calculated by adding input work 120 to the enthalpy of saturated liquid 10.
The efficiency of any thermodynamic power cycle, including the Rankine cycle, can be described by the ratio of the total work output divided by the input thermal energy. In addition, the waste heat ratio can be described by the ratio of waste heat expelled divided by the input thermal energy.
For prior art Rankine cycle 500, the total output work is the difference between output work 200 and input work 120. However, input work 120 is much smaller than output work 200 and as a result, can be neglected. This means the total work can be closely approximated by output work 200. As a result, the efficiency for prior art Rankine cycle 500 the efficiency can be closely approximated by the ratio of output work 200 divided by input thermal energy 100. And, the waste heat ratio is waste thermal energy 150 divided by input thermal energy 100. The sum of the efficiency and the waste heat ratio is substantially one. For prior art Rankine cycle 500 with a high temperature source for input thermal energy 100, thermodynamic efficiencies can range from 0.3 to 0.4; with corresponding waste heat ratios ranging from 0.7 to 0.6. For low temperature energy sources, such as geothermal and solar thermal, the resulting thermodynamic efficiencies for prior art Rankine cycle 500 are less than 0.12; with corresponding waste heat ratios of greater than 0.88. This shows that the higher the temperature of input thermal energy 100 the greater the thermodynamic efficiency. This is because a larger portion of input thermal energy 100 is converted to output work 200 and a smaller portion of the energy is expelled as waste energy 150.
A Carnot cycle is an ideal power cycle that has the highest possible theoretical thermodynamic efficiency. In a Carnot cycle, heat is exchanged between a high temperature reservoir and a working fluid and heat is exchanged between a low temperature reservoir and a working fluid to produce work. The thermodynamic efficiency of a Carnot ηc cycle is described as
      η    ⁢                  ⁢    c    =      1    -          Tl      Th      where TI is the absolute temperature of the low temperature reservoir and Th is the absolute temperature of the high temperature reservoir. Even though prior art Rankine cycle 500 efficiency is less that the Carnot cycle efficiency, a comparison between the Carnot cycle efficiency and prior art Rankine cycle 500 efficiency for identical temperature limits show the same type of relationship between the temperatures and cycle efficiencies. For example, a representative high temperature source for input thermal energy source 100 is 773° K and a representative temperature for saturated vapor 16 is 333° K. The temperature input thermal energy 100 corresponds to the high temperature reservoir and the temperature of saturated vapor 16 corresponds to the low temperature reservoir. Substituting these temperatures into the Carnot cycle efficiency equation produces an efficiency of 0.56. For representative low temperature sources for input thermal energy 100 is 523° K. If saturated vapor temperature 16 remains the same, the Carnot cycle efficiency is 0.36.
A means to increase the efficiency of a thermodynamic power cycle including a Rankine cycle would be beneficial. Prior art methods for increasing efficiencies rely on recovery of the waste heat from a primary, “top”, power cycle as input thermal energy to a, “bottom”, power cycle, as disclosed in U.S. Pat. Nos. 9,206,710, 9,145,795, 9,115,603, 9,051,852, 9,021,808 and 9,003,798. The working fluid for these low temperature power cycles is typically a low vaporization temperature organic fluid. The thermodynamic efficiencies of these, “bottom”, power cycles are subject to the same limitation as that of the, “top”, power cycles. That is, a larger portion of the energy is expelled as waste heat. In addition, because of their low temperatures, the bottom cycle thermodynamic efficiencies are limited to approximately 12 %. As a result, the overall efficiency gains made possible by additions of bottom power cycles are limited to small fractions of the input thermal energy. This means that small thermodynamic efficiency increases can be achieved through the recovery of the heat of condensation.
A means to increase the input thermal energy without increasing the waste heat would increase the thermodynamic efficiency of the power cycle. This approach for increasing the thermodynamic efficiency would be beneficial.